1. Field of the Invention
The present invention relates to frequency modulation ("FM") demodulators, and in particular, FM demodulators with adjustable resonant frequencies of operation.
2. Description of the Related Art
Various forms of frequency demodulators, or discriminators, for demodulating FM signals are well known in the art. A common type of frequency demodulator which is used in the demodulation of a digitally modulated FM signal includes a mixer interconnected with a phase shifter. The phase shifter and one input of the mixer both receive the modulated signal. The output of the phase shifter, which is a phase-shifted replica of the inputted modulated signal, is received by the other input of the mixer. These two signals are mixed within the mixer and thereby produce a low frequency output signal which corresponds to the instantaneous frequency deviation of the inputted modulated signal.
The phase shifter is typically some form of a resonant element or structure, such as a series or parallel resonant circuit made up of inductors and capacitors (e.g. a quadrature tank circuit). Such reactive components will have values which are dependent upon temperature. This results in the resonant frequency of the circuit being temperature-dependent, which in turn causes the resulting phase shift to also be temperature-dependent.
When using this type of demodulator circuit for receiving digitally modulated FM signals, it is important that the voltage swing of the mixer output signal be maximized. This can be achieved by using a phase shifter with a high circuit Q value, provided that the resonant frequency of the phase shifter is correct. If the resonant frequency shifts with temperature, or if the center, or carrier, frequency of the incoming FM signal shifts, the output voltage swing can rapidly decrease because of the narrowness of the bandwidth of the phase shifter due to its high circuit Q value.
Accordingly, it would be desireable to have a frequency demodulator which has an adjustable resonant frequency to allow for compensation of phase shifts caused by temperature variations or shifts in the carrier frequency of the incoming FM signal.